This type of touch-sensing system is known as an FTIR-based system (FTIR, Frustrated Total Internal Reflection). It may be implemented to operate by transmitting light inside a solid light-transmissive panel, which defines two parallel boundary surfaces connected by a peripheral edge surface. Light generated by a plurality of emitters is coupled into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces to a plurality of detectors. The light thereby defines propagation paths across the panel, between pairs of emitters and detectors. The emitters and detectors are arranged such that the propagation paths define a grid on the panel. An object that touches one of the boundary surfaces (“the touch surface”) will attenuate (“frustrate”) the light on one or more propagation paths and cause a change in the light received by one or more of the detectors. The location (coordinates), shape or area of the object may be determined by analyzing the received light at the detectors. This type of apparatus has an ability to detect plural objects in simultaneous contact with the touch surface, known as “multi-touch” in the art.
There are different ways of coupling light into and out from the panel. One way is to attach prisms or wedges to the bottom surface of the panel. Such solutions have the benefit that light may be coupled within controllable angles, and it also allows a flush, edge-to-edge solution, for the top surface. However, the prisms or wedges may add significant thickness and weight to the system.
In another configuration, e.g. disclosed in US2006/0114237, the light is coupled into the panel directly through a peripheral side edge surface. Such an approach allows the light to be simply and efficiently injected into the panel. Also, such an in-coupling does not add significantly to the thickness of the touch system. On the other hand, in-coupling via the edge surface may add to the footprint of the touch system. Furthermore, mounting and calibration of light emitters and detectors with respect to the side edge may be difficult in order to obtain an acceptable production yield.